Method and device for testing machine tool

ABSTRACT

On a machine tool in which a first element intended to support a machining tool, and a second element intended to support a workpiece, are mutually movable, a special test arrangement may be placed between the first element and the second element . During a mutual displacement between the first and second elements, this test arrangement can apply a predetermined force between these elements while at the same time making it possible to record the resulting shift between them. The measured results may be used for analysis of the machine&#39;s condition.

TECHNICAL FIELD

The present invention relates partly to a method and apparatus fortesting a machine tool.

STATE OF THE ART

In many contexts, industry uses various types of machine tools forprocessing and manufacturing various parts. This involves needing to beable to monitor the status of such machines in order, for example, to beable to do repairs and adjustments in time to avoid stoppages or badprecision of parts being manufactured. The objective is to be able todetect changes quickly and be able to rectify them before major andexpensive defects develop.

A conventional method for testing a machine is to make representativeparts and then check their dimensions in order to assess the machine'sperformance. Disadvantages observed in this respect include the need touse tools and testpieces and difficulty in comparing results fromdifferent tests. Using standardised testpieces certainly facilitatescomparisons but still requires testpieces and tools.

Another method is to monitor the machine's rigidity in differentdirections by applying a suitable force by means of a hydraulic cylinderand measuring the resulting deflection by means, for example, of amicrometer. A disadvantage of this method is that it is time-consumingand measurement cannot be done with the machine in operation.

Another known practice is to use a special instrument, a so-called “ballbar”, fitted between workpiece holders and tool holders, to test themachine's ability to perform a circular movement. Measuring equipment inthe instrument is used to record any deviations from a circle. Varioussuch tests can be carried out at different times and compared to provideinformation on various characteristics of the machine, such ascircularity, servo response, rectilinearity, play etc. Tests can also bedone at various feed rates, in various feed directions and using bars ofvarious lengths, and placing the workpiece holder at various differentpoints.

There nevertheless remains the disadvantage of it not being easy to gaina proper assessment of the machine's characteristics under load.

OBJECTS OF THE INVENTION

The object of the invention is to make better machine testing possible.A further object is to achieve this by simple means.

DESCRIPTION OF THE INVENTION

The object of the invention is achieved by a method and an apparatus ofthe invention. On a machine tool in which a first element intended tosupport a machining tool, and a second element intended to support aworkpiece, are mutually moveable, a special test arrangement may beplaced between the first element and the second element. During a mutualdisplacement between the first and second elements, this testarrangement can apply a predetermined force between these elements whileat the same time making it possible to record the resulting shiftbetween them. The measured results may be used for analysis of themachine's condition.

Applying a predetermined force between the first and second elementsduring mutual displacement between them, and simultaneously measuringthe resulting deformation, makes it possible to carry out machinetesting in much more production-like conditions than was previouslypossible. Analysis can be further refined by also varying the manner inwhich the displacement takes place, as regards both movementconfiguration and direction of movement, and also by varying themagnitude of the force applied.

Further features and advantages of the invention are described in moredetail with reference to embodiments depicted in the attached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view of a machine tool with an arrangementaccording to the invention,

FIG. 2 is a view from above of an arrangement according to theinvention,

FIG. 3 is a side view, partly in section, of the arrangement in FIG. 2,

FIGS. 4 and 5 show deflection in various directions between machineelements at various loads, and

FIG. 6 is a load diagram.

DESCRIPTION OF A PREFERRED EMBODIMENT

FIG. 1 depicts a machine tool 1 intended for the machining of workpiecesby means of a tool fitted in the machine. This machine incorporates afirst element 2 in the form of a spindle, and a second element 3 in theform of a worktable, which are movable relative to one another in aconventional manner in various directions for machining of a workpiece(not depicted) which is intended to be secured to the worktable and bemachined by a tool inserted in the spindle. The first element 2 issupported by a spindle head 4 which is itself supported by a frame 5which also supports the second element 3. To test how the machine 1behaves during mutual displacement of the first element 2 and the secondelement 3, a test arrangement 6 described below and designed accordingto the invention is clamped between the first element 2 and the secondelement 3.

According to FIGS. 2 and 3, the test arrangement 6 incorporates aconventional measuring arm 7 of the “ball bar” type, which has one ofits ends linked by an articulation 8 to a bracket 9 intended to besecured in the first element 2 of the machine 1, and has its other endlinked via an articulation 10 to a bracket 11 intended to be fixedrelative to the second element 3. A telescopic element 12 links the twoarticulations 8 and 10 and is provided internally with measuringequipment (not depicted) for recording changes in the distance betweenthe articulations 8 and 10. The measuring arm 7 and its measuringequipment can be connected by an electrical line 13 to suitable externalequipment 30 for recording and analysis of measurement results. Inparticular, the actual measured mutual relative paths of the first andsecond elements are compared in the equipment 30 with an intendedrelative movement path to provide an indication of the machine'scondition.

The test arrangement 6 also incorporates a power unit 14 which, like themeasuring arm 7, is intended to be fixed between the first element 2 andthe second element 3. For this purpose there is not only a bracket 15for fixing to the first element 2 but also a bracket 16 for fixing tothe second element 3. Via a bearing 17 the bracket 16 supports apivotingly mounted arm 18 which has its free end connected to a yoke 19in which piston rods 20, 21 to two working cylinders arranged in acylinder housing 22 are fastened. The cylinder housing 22 is providedwith working medium via a line 23 and is fixed to the holder 15 which isintended to be pivotable about the first element 2 via a bearing 24.

The power unit 14 includes at least one working cylinder that can beused to apply a force substantially parallel with the measuring arm andin either opposite direction, parting or drawing together the twobrackets 15 and 16, and hence also the first element 2 and the secondelement 3, when the test arrangement 6 is fitted for use according toFIG. 1. The measuring arm 7 can at the same time be used to ascertainthe magnitude of the resulting displacements between the elements 2 and3.

The results of a test series are depicted schematically in FIG. 4, inwhich the machine 1 was programmed to cause the second element 3 toperform a circular movement about the first element 2 duringsimultaneous application of force between the elements 2 and 3. in thisdiagram, a coordinate system with X and Y axes has been placed with itscentre 24 in the first element 2, and curves a-f show the magnitude ofthe deflection in various positions resulting from various amounts offorce. In the case of curves b, d and f, provided with arrows, themovement, viewed from above in FIG. 1, was in a clockwise direction,while that depicted by the other curves (a, c and e) was in ananticlockwise direction. The force applied was 330N on curves a and b,660N on curves c and d and 825N on curves e and f. The feed rate, i.e.the circumferential speed of the second element 3 with respect to thefirst element 2, was 1000 mm/mm in all cases. As may be seen, the amountof displacement increases with the amount of force applied butdifferently in different directions of movement.

FIG. 5 shows schematically the results of a test series in somewhatdifferent conditions from the test series in FIG. 4. In this case thefeed rate was increased to 5000 mm/min. The force applied for curves a-dwas the same as in FIG. 4 but was increased to 990N for curves e and f.Here again the amount of deflection increases with the amount of forceapplied and differs in different directions, but not in the same way asin FIG. 4.

The diagrams in FIGS. 4 and 5 may be said to constitute the machine's“fingerprint” in various situations and provide, inter alia, informationon how its rigidity varies in different directions at different amountsof load. They also provide a picture of the precision with which themachine can perform a certain type of movement under given conditions.

Another type of test result is depicted in FIG. 6, where the change inrectilinearity S, measured in micrometers, is shown as a function of themagnitude of the force applied F, indicated in N (newtons). It showsthat the change is linear.

It is obvious that further types of tests may provide further types ofinformation about the machine. It is possible, for example, in aspecified mutual position between the first element 2 and the secondelement 3, to carry out measurements both with force increase and withforce decrease in order to gain an assessment of hysteresis in themeasuring system. This makes it possible to find out how to compensateinternal friction and elasticity in the measuring system with a view toimmediately produce reliable measuring results.

For precision reasons, the bearings 17 and 24 used must have goodaccuracy, i.e. minimum play, and, at the same time, low friction.Sliding or rolling bearings, e.g. needle bearings, may be suitable forthe purpose but magnetic or hydrostatic bearings are also attractive,although expensive.

The movements described above were in a substantially horizontal planebut there is of course nothing to prevent analysis of movements in aplane with a different orientation, e.g. vertical. An improved versionof the conventionally made articulations 8 and 10, with a view toenabling them to absorb sufficiently large forces, may make it possibleto integrate the power unit 14 with the telescopic element 12. It isalso possible at the same time to make the articulations 8 and 10movable in three dimensions in order to be able to carry out testing inany desired direction. An integrated version makes for easier fittingand removal.

Signal transmission from the measuring arm 7 is here depicted via anelectrical line 13 but other versions are of course conceivable, e.g.using some kind of wireless transmission to avoid problems withelectrical lines during rotary movements.

In the case of the machine tool depicted in FIG. 1, the first element 2is fixed while the second element 3 is movable in a plane perpendicularto the plane of the drawing. There is of course nothing to prevent thesecond element 3 being fixed instead while the first element 2 may bemovable. A combination of such movements is also possible, depending onwhat is necessary and desirable in the particular case.

The power unit 14 described above may within the scope of the inventionalso take a number of different forms, e.g. it is possible for the arm18 to take the form of a cylinder housing instead. It is also possiblefor a single cylinder to be used instead of two, etc.

As previously indicated, the two brackets 15 and 16 are to make itpossible to use the power unit 14 to load the first element 2 and thesecond element 3, so said brackets have within the scope of theinvention to be adapted as appropriate to the particular type ofmachine.

Diagrams of the type depicted in FIGS. 4-6 may be used for calculating alarge number of different parameters which characterise the machine'sbehaviour under load. Precision in circular movement can be read off,but precision in linear movements and various types of compositemovement can also be calculated. The amount of force dependency providesa good measure of the machine's quality in that little force dependencyindicates good quality and good precision, whereas great forcedependency indicates less good quality and inferior precision.

In addition, the measured values arising from testing a certain machinetool according to the invention may be used for imposing corrections torectify deflection in various load situations in the control programmefor the machine concerned. The machine's accuracy might thus besubstantially improved. The economic gains might become significant ifrelatively inexpensive machines could therefore be used instead of moreexpensive high-precision machines. Said measured values obtained mayalso be used in digital simulation of the actual machining process toprovide a more realistic picture of that process.

What is claimed is:
 1. A method for testing a machine tool, wherein themachine tool includes a first element for supporting a machining tool, asecond separate element for supporting a workpiece and the first andsecond elements are relatively moveable to various settable patterns ofmovement; a measuring arm between the first and second elements andmeasuring the relative patterns of movement; the method comprising:displacing the first and second elements mutually in an intended mutualcircular movement path, and applying a predetermined force between thefirst and second elements and substantially in the longitudinaldirection of the measuring arm in various mutual positions between thefirst and second elements; recording the actual movement path by themeasuring arm; comparing the intended movement path with the actualmovement path for determining the machine condition.
 2. The method ofclaim 1, further comprising continuously applying the predeterminedforce during the mutual displacement between the first and secondelements.
 3. The method of claim 2, wherein the magnitude of thepredetermined force is periodically altered and the actual relativemovement between the first and second elements is thereafter repeatedwith recording of the movement paths and subsequent comparison with theintended movement path.
 4. The method of claim 1, wherein the intendedmutual circular movement path includes circular movements, and aplurality of the circular movements are performed in opposite rotationaldirections.
 5. The method of claim 4, wherein the circular movements areperformed at different radii.
 6. The method of claim 1, wherein theintended mutual circular movement path includes circular movements andthe circular movements are performed at different radii.
 7. The methodof claim 6, wherein the intended mutual circular movement paths comprisesimilar patterns of movement performed at different machine feed rates.8. The method of claim 1, wherein the intended mutual circular movementpaths comprise similar patterns of movement performed at differentmachine feed rates.
 9. The method of claim 8, wherein the patterns ofmovement are performed with a feed rate which is normal for the machine.10. The method of claim 1, wherein the intended mutual circular movementpath comprises a plurality of the intended paths and the movement pathsin one plane.
 11. The method of claim 1, wherein the intended mutualcircular movement path comprises a plurality of the intended paths andthe actual movement paths are performed in three dimensions.
 12. Themethod of claim 1, wherein the actual movement paths are performed inthree dimensions.
 13. The method of claim 1, wherein the magnitude ofthe predetermined force is periodically altered and the actual relativemovement between the first and second elements is thereafter repeatedwith recording of the movement paths and subsequent comparison with theintended movement path.